Micro-Raman imaging of GaN hexagonal island structures

Abstract

We use Raman scattering to obtain a stress map of lateral epitaxy overgrown GaN. Isolated hexagonal islands are grown by selective area overgrowth without a seed layer. Stress mapping is obtained from shifts in the E2 phonon. GaN in the aperture area has the greatest biaxial compressive stress, ≈0.18 GPa. The overgrowth region is under slightly smaller stress, about 0.15 GPa. We attribute marked variations in the A1(LO) phonon intensity to spatial variations in the free carrier concentration. This is found to be small in the aperture region and high in the lateral overgrowth. The position-dependent presence of the lower coupled plasmon–phonon band is consistent with this interpretation.